1. Field of the Invention
The present invention relates to an amplification circuit, and more particularly, to an amplification circuit capable of adaptively controlling bias current.
2. Description of the Prior Art
An operational amplifier is widely used for realizing a variety of circuit functions. Take driving circuits of a liquid crystal display (LCD) for example, the operational amplifier can be used as an output buffer, which charges or discharges loading ends, i.e. liquid crystals, according to analog signals outputted by a front stage digital-to-analog converter (DAC), for driving corresponding pixel units on the LCD. However, with increases in sizes and resolutions of LCDs, the amount of data processed by the driving circuits per unit of time is also increasing significantly, so that response speed of the operational amplifier, also called slew rate, has to be enhanced as well.
The operational amplifier generally has a three-stage structure, which includes an input stage circuit, again stage circuit, and an output stage circuit. The input stage circuit is utilized for increasing input impedance of the operational amplifier, the gain stage circuit is utilized for increasing current or voltage gain of the operational amplifier, and the output stage circuit is utilized for driving capacitive or resistive loads connected to the operational amplifier. In addition, since the operational amplifier may suffer loop instability problems, Miller compensation capacitors are commonly implemented to perform frequency compensation for improving loop stability. Moreover, for driving external loads connected to the operational amplifier, the drive current of the output stage circuit is generally greater than bias current of the input stage circuit. In this case, when the operational amplifier drives a heavy load, the slew rate is often restricted by the bias current of the input stage circuit.
In general, the response speed (slew rate) of the operational amplifier is decided by the bias current of the input stage circuit and the driving capability of the output stage circuit. In this situation, the response speed (slew rate) of the operational amplifier can be expressed by the following slew rate equation:
      SR    =                  I        C            =                        Δ          ⁢                                          ⁢          V                t              ,in which “I” indicates a bias current, “C” indicates capacitance of the internal capacitors, and “ΔV” indicates voltage variation of the output voltage of the operational amplifier. This means that the response speed of the operational amplifier is decided by the charging (or discharging) speed when the internal capacitor of the operational amplifier is charged (or discharged) by the bias current of the input stage circuit. As can be seen from the above, when the bias current of the input stage circuit increases, the internal capacitors can be charged or discharged much faster, so the response speed of the operational amplifier can be enhanced as well. Therefore, in the prior art, the internal slew rate of the operational amplifier is generally enhanced by increasing the bias current of the input stage circuit so as to increase the driving speed of the operational amplifier.
Therefore, the prior art increases the bias current of the input stage circuit in a fixed period during operation of an operational amplifier to enhance the response speed of the operational amplifier. However, due to variations in pressure-volume-temperature (PVT) in actual application, an operational amplifier may have different enhancement effects of slew rate indifferent operating environments. Also, different operational amplifiers may have different enhancement effects of slew rate while operating in the same operating environment due to individual differences. In such situations, the method used by the prior art may introduce some problems.
For example, for an operation amplifier with a small internal capacitor, even after the internal capacitor has been sufficiently charged, the enhanced bias current continues to be provided to the operational amplifier, resulting in water of power consumed until a fixed period ends. This is disadvantageous to electric devices requiring low power consumption. Conversely, for an operation amplifier with a large internal capacitor, if the enhanced bias current is provided only for a fixed period, the internal capacitor cannot store enough electric charges within the fixed period, causing a bad enhancing effect of the slew rate and insufficient driving capability.
In view of above, the prior art blindly increases the bias current of the operational amplifier in a fixed period to enhance the slew rate without making appropriate adjustments to adapt to different operating environments and device requirements.